A tire test (tire uniformity test) is conventionally conducted in which the state or quality such as uniformity (consistency) of a tire, as a finished product, is measured to determine whether the tire is bad or good. In the tire test, the tire is pressed against the outer circumferential surface of a load drum that is mounted on a tire uniformity machine. In this situation, a load applied in radial and lateral directions of the tire, when the tire is caused to rotate, is measured as a uniformity waveform using a device such as a load cell mounted to the load drum, and tire uniformity is assessed, based on the measured uniformity waveform.
Parenthetically, a load drum mounted to the tire uniformity machine is machined into, for example, a cylindrical shape whose cross-section is a true circle; however, the cross-section is not strictly made to be a perfect true circle because of limitations of machining accuracy or the like. In other words, some unevenness is inevitably created on the outer circumferential surface of the load drum. In this way, when a tire is caused to rotate with the tire in contact with the outer circumferential surface of the load drum which is not the perfect true circle, rotation vibrations are created at a rotation shaft of the load drum when the tire passes on the slight unevenness that exists on the outer circumferential surface of the load drum. The created rotation vibrations are contained as an error in a uniformity waveform measured with a load cell. As a result, the accuracy of the tire uniformity calculated based on a uniformity waveform containing such an error is likely to degrade.
Consequently, in order to remove an error resulting from the rotation vibrations of such a load drum from the uniformity waveform measured with a device such as a load cell, in other words, to correct the measured uniformity waveform, some methods of correction have already been conceived (Patent Document 1, Patent Document 2, etc.).
In the method of correction in, for example, Patent Document 1, a detector (sensor) is provided which is capable of detecting a displacement of the outer circumferential surface of the load drum (displacement in the radial direction of the tire, or alternatively that in the lateral direction thereof). And the displacement of the load drum detected with the detector is measured as rotation vibrations. Then, what is obtained by multiplying the measured load drum's rotation vibrations by a tire's spring constant is computed as a waveform of force variation exerted by rotation vibrations on the load drum. The uniformity waveform can be corrected by subtracting the thus computed waveform of force variation, as a correction waveform for correcting an error resulting from the rotation vibrations, from the actually measured uniformity waveform.
Further, in the method of correction in Patent Document 2, a uniformity waveform measured with a load cell is divided into a data segment per revolution of a tire, and the divided uniformity waveforms are overlaid together. In this way, if uniformity waveforms are overlaid together for each data segment per revolution of the tire, then the error can be canceled by overlaying, and an average uniformity waveform can be obtained. Subtracting such an average uniformity waveform from the uniformity waveform actually measured with the load cell, yields a waveform that contains an error component resulting from the rotation vibrations of the load drum. The yielded waveform containing the error component is next divided into the data segment per revolution of the load drum, and when the divided waveform segments are overlaid together, the waveforms containing the error components are averaged. As with Patent Document 1, this enables finding a correction waveform for correcting the error resulting from the rotation vibrations.